1. Field of the Invention
The present invention relates to a solid-state imaging device having a plurality of photodetecting portions that are arranged two-dimensionally and vertical CCD registers and a horizontal CCD register(s) for transferring signal charges that are output from the photodetecting portions, as well as to a driving method of such a solid-state imaging device.
2. Description of the Related Art
Among conventional CCD imaging devices of the above kind is known one having a horizontal CCD register shown in FIGS. 1A and 1B and FIGS. 2A–2E.
In the CCD imaging device shown in FIG. 1A, one horizontal CCD register 20 is provided for a plurality of vertical CCD registers 10 on the horizontal CCD register 20. Two-layer transfer electrodes 30A and 30B are provided in two pairs for each vertical CCD register 10 on the horizontal CCD register 20. Two-phase horizontal drive pulse signals H1 and H2 having opposite polarities are applied alternately to the pairs of transfer electrodes 30A and 30B.
As shown in FIG. 1B, the charge transfer region of the horizontal CCD register 20 is so formed as to have prescribed potential differences that correspond to the respective transfer electrodes 30A and 30B.
Therefore, by applying the horizontal drive pulse signals H1 and H2 to the transfer electrodes 30A and 30B corresponding to each pair of vertical CCD registers 10 adjacent to each other as shown in FIGS. 2A and 2E, a signal charge transfer operation is performed in such a manner that the potentials of the charge transfer region of the horizontal CCD register 20 are controlled sequentially as shown in FIGS. 2B–2D.
That is, a two-phase-drive CCD, in which phase control can be performed relatively easily, is employed as the horizontal CCD register 20 that transfers signal charges at high speed.
FIGS. 3A–3F show another conventional CCD imaging device in which two horizontal CCD registers are provided for a plurality of vertical CCD registers.
Specifically, in the CCD imaging device shown in FIG. 3A, two horizontal CCD registers 50 and 60 are provided for a plurality of vertical CCD registers 40. The two horizontal CCD registers 50 and 60 share handling of signal charges that are output from the respective vertical CCD registers 40 and perform transfer operations parallel. Signal charges that are transferred by the horizontal CCD registers 50 and 60 are output to a charge detecting section (not shown) via a final gate section 70.
Also in this CCD imaging device, in each of the horizontal CCD registers 50 and 60, two-layer transfer electrodes 80A and 80B are provided in two pairs for each vertical CCD register 40. Two-phase horizontal drive pulse signals H1 and H2 having opposite polarities are applied alternately to the pairs of transfer electrodes 80A and 80B. Signal charges are transferred by the same operation as in the CCD imaging device of FIG. 1A and merge together in the final gate section 70.
Incidentally, in image sensors using the above conventional CCD imaging devices, the signal output rate should be increased as the number of pixels increases if the number of frames per unit time is kept constant. Actually, however, the output rate can not necessarily be increased easily when other factors such as the power consumption are taken into consideration.
On the other hand, when the output rate is not increased, the number of frames per unit time should necessarily be decreased, which means decrease in the amount of information relating to motion.
In still cameras, for example, it is necessary to bring the camera into focus and determine exposure by monitoring images in addition to take images to be recorded actually. For merely monitoring images, it is more important to secure a sufficient number of frames per unit time than obtaining high-resolution images.
In view of the above, a conventional technique is employed that the number of pixels used is decreased by decimating pixels and the number of images per unit time is thereby increased.
Among methods for decimating pixels are a method that read operations themselves on the photodetecting portions are decimated and a method that the number of signal charge packets is decreased by mixing signal charges adjacent to each other.
Pixels can be decimated in the vertical direction by prohibiting reading from part of the photodetecting portions through control by the read gate. However, it is difficult to perform reading for only arbitrary pixels in the horizontal direction because the electrodes are arranged continuously in the horizontal direction.
Therefore, in the horizontal direction, the method that signal charges are mixed with each other remains a candidate. However, the number of transfer stages is fixed in ordinary structure horizontal CCD registers as shown in FIG. 1A. This results in a problem that the drive frequency cannot be decreased in accordance with a decreased number of signal charge packets and hence the frame rate cannot be increased.